Field of the Invention
Embodiments of the present invention relate generally to a charged particle beam writing apparatus and a charged particle beam writing method, and more specifically, for example, relate to a method for correcting chromatic aberration of an electron beam used in an electron beam writing apparatus that irradiates electron beams on a target object.
Description of Related Art
In recent years, with high integration of LSI, the line width (critical dimension) of circuits of semiconductor devices is becoming progressively narrower. As a method for forming an exposure mask (also called a reticle) used to form circuit patterns on these semiconductor devices, the electron beam (EB) writing technique having excellent resolution is employed.
FIG. 19 is a conceptual diagram explaining operations of a variable-shaped electron beam writing or “drawing” apparatus. The variable-shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular aperture 411 for shaping an electron beam 330. A second aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330, which was emitted from a charged particle source 430 and has passed through the aperture 411, is deflected by a deflector to pass through a part of the variable shape aperture 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., x direction) during writing. In other words, a quadrangular shape that can pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) system.
In electron beam writing, importance is attached to throughput, in manufacturing masks. On the other hand, importance is attached to resolution of beams, in performing various evaluation to develop next-generation lithography, because finer pattern formation is requested therefor.
In terms of putting emphasis on throughput, when increasing luminance of the cathode of an electron gun, it is necessary to increase an emission current emitted from the cathode of the electron gun. Therefore, required is a writing apparatus which can deal with (respond to) both the high emission condition (high emission mode) attaching weight to throughput, and the low emission condition (low emission mode) attaching weight to resolution. However, if the emission current is increased, so-called chromatic aberration becomes worse due to energy dispersion increase by the longitudinal Boersch effect. Accordingly, there is a problem that the states of chromatic aberration in both the modes are different. This results in that the states of chromatic aberration of electron beams illuminating the shaping aperture arranged at the downstream side become different and shaped beams are affected by such chromatic aberration.
With respect to adjustment of an illumination optical system, there is disclosed a technique for flattening a current density of a beam at the shaping aperture which shapes beams at the downstream side, wherein the flattening is performed by moving a trim aperture up and down to adjust the positional relationship between the trim aperture and the crossover of the beam immediately beneath the trim aperture (for example, refer to Japanese Patent Application Laid-open (JP-A) No. 2007-208035).